In rotary drilling a well such as a hydrocarbon well, a drill bit is rotated in the well by means of a drill string composed of pipes linked end-to-end. The drill string is rotate either by means of a kelly mounted at its upper end and engaging a driven rotary table or by connection directly to a motor and gear box arrangement. A drilling fluid, often known as "mud" is pumped, through the kelly when present, into the drill string where it passes through the bit and back to the surface in the annular space between the drill string and the borehole wall. At the surface the fluid passes through solids control equipment to holding tanks from which it is drawn by pumps for injection into the drill string as described. The mud serves various purposes including stabilisation of the borehole and balancing of pressurized formation fluids, especially gas. If the hydrostatic pressure exerted by the mud is too low, high pressure gas in the formation being drilled can enter the borehole and pass up the annulus in the mud. This influx or "kick" is potentially very dangerous as the presence of the gas in the annulus further reduces the hydrostatic pressure exerted on the formation by the mud due to the decrease in apparent density. Further influxes of gas can then occur leading to a potential blow-out and loss of control of the well. Thus it will be appreciated that early detection of influxes is essential if the drilling operation is to proceed safely. Because an influx will displace mud, surface monitoring of mud flow rates or mud tank levels has been proposed to detect influxes.